In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesired moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. Moisture levels inside a cable sheath system may have a detrimental effect on the transmission characteristics of the cable.
Furthermore, water may enter the cable because of damage to the sheath system which compromises the integrity of the cable. For example, lightning, rodent attacks, or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter and, if not controlled, to move longitudinally within the cable.
Due to the possibility of transmission degradation caused by the presence of water within a cable, cables for transmitting communications signals must meet industry standards with respect to water blocking provisions. For example, one industry standard requires that there be no transmission of water under a pressure head of one meter in one hour through a one meter length of cable.
Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not necessarily detrimental to the performance of optical fibers in the cable, passage of the water within the cable interior to connection points or terminals or associated equipment inside closures, for example, may cause problems, especially in freezing environments and should be prevented.
Filling materials have been used to fill cable cores and atactic polypropylene or other flooding materials have been used to coat portions of cable sheath systems such as the outer surface of a metallic shield, for example, to prevent the longitudinal movement of any water which enters the cable. Although the use of a filling material, generally in the form of a grease or gel-like substance, has limitations, such as causing housekeeping problems, inhibiting manufacturing line speeds because of the need to fill carefully interstices of the cable core and presents problems for field personnel during splicing operations, it continues to be widely used to prevent entry of the water into the core.
As an alternative to the use of filling material as the method of water-blocking, some arrangements of communications cable use one of several forms of longitudinally extending members inside the core tube. Presently, many commercially available cables include a water-swellable tape. The tape is used to prevent the travel of water through the sheath system and into the core as well as its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, and includes a water-swellable powder which is positioned between two non-woven tissues. Although such a tape provides suitable water protection for the cable, it is relatively expensive and thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware.
As a solution to some of the foregoing problems, prior art systems have incorporated a water-blocking member in the form of a strip or a yarn which covers only a portion of an inner periphery of the cable. In this way, the strip or the yarn separates only a portion of the jacket from other portions of the sheath system. If adhesion between the jacket and the other portions of the sheath system is desired, that adhesion is not compromised by the water-blocking member. Further, such a strip or yarn is less expensive than one which covers substantially an entire inner periphery of the cable.
Even with the various alternative methods and physical arrangements presently used to prevent the ingress of water through various sections of a communications cable, there still exists a need for a reliable and relatively inexpensive technique for prohibiting water flow within a cable. The use of either a hydrophobic filling material exclusively or a hydrophilic yarn arrangement exclusively do not consistently satisfy the ever-tightening industry standards for water-blocking communication cables.
In general, the use of only water-absorptive yarns does not adequately eliminate all of the voids and interstices created between the various portions of the cable core. This is true even when the yarns are separately wrapped around each of the sections of the cable core. Since the yarn generally is supported by one particular section of the cable core which often has a substantially circular cross-section, interstitial voids exist as the various sections are brought into physical contact with each other. In a copper cable, the various sections of the cable core desired to be wrapped may include a number of 1) individual twisted pair conductors, 2) twisted pair units which each include a plurality of twisted pair conductors, or 3) the entire cable core which generally includes multiple units and twisted pairs.
Furthermore, the use of only water-repelling materials does not adequately fill all of the voids and interstices created between the various portions of the communications core. Even using some of the known high pressure, high temperature methods of inserting gel-like materials to fill a series of crevices does not totally fill each of the desired areas. Furthermore, when the cable is subjected to various forces, such as bending, during manufacturing and installation, the material located within the communication core is often caused to shift, thereby creating new openings through which water may flow longitudinally along the cable. Yet another disadvantage with the use of only water-repelling materials to prevent water movement along a cable is that most, if not all, of the known filling compounds shrink due to thermal cooling after being inserted into the cable. This shrinkage of the filling compound causes additional voids or interstices to be created with no additional means of preventing water flow through the newly created openings.
To date, due to the basic, but drastic, differences in operation of hydrophobic materials and hydrophilic materials, practitioners have purposefully avoided using these two compounds in direct cooperation with each other. Skilled artisans have stated a belief that the operational properties of each type material would unacceptably hinder the effectiveness of the other material.
What is needed and seemingly what is not available is a communications cable which includes increased protection against the flow of water along the cable. The sought-after cable should appropriately incorporate at least two different types of materials to prevent water flow along the cable, specifically both a hydrophobic material in cooperative communication with a hydrophilic material.